Construction of light box communication device for clients with severely limited communication skills.
The box has a translucent cover with two lamps underneath, such that one side or the other of the box may light up. A control circuit provides switch input detection, lamp switching, lamp pre-warming, and lamp voltage reduction.
(Click on images for higher resolution view)
State | Left Lamp | Right Lamp |
| One | Warming (5%) | Warming (5%) |
| Two | On (80%) | Warming (5%) |
| Three | Warming (5%) | On (80%) |
The software also implements a three minute timeout, such that it will automatically revert to the initial state (both lamps at 5%) if no input is detected for three minutes. This is to prevent heat build-up within the case, which might otherwise deform the perspex cover.
A pair of fans are glued to the base angled such that they disturb and redistribute the heat plume from each lamp.
See also the assembly language source file that was compiled using gpasm and written into the 12C509 chip.
The unit uses a 12VDC 2A unregulated supply from Jaycar. This generates some 20 volts DC when unloaded, and drops to about 15 volts under single lamp load. A two amp PCB mounted glass fuse provides fault protection, followed by a 1N5404 diode as reverse polarity protection.
The supply after the diode is distributed to the cooling fans, the logic
regulator, and the lamps. A second diode, 1N4148, separates this supply from
the logic regulator's 1000uF capacitor, to prevent lamp switching power being
supplied by the capacitor. The LM7805 voltage regulator provides a stable 5V
supply for the microcontroller. A pair of capacitors on the 5V bus quench
switching transients from the microcontroller.
The unregulated supply is applied to two 12V fans, in series, with a 100 ohm
resistor and two 1N4004 diodes. The fans are operated at about 6V each, running
slow and quietly by comparison to their top speed.
The R/C network attached to the trigger switch acts to (a) reduce contact
bounce, and (b) increase resistance to induced noise. Without R3 (100 ohm), the
switch had been found to short the capacitor, inducing a reset on the
microcontroller.
Two outputs from the microcontroller connect directly to the gates of two 2SK2175 logic level power MOSFETs. When +5V is applied to the gate, this type of MOSFET will conduct via source and drain with a resistance below 0.1 ohm, lighting the lamp.
A prototype circuit used MTP3055E MOSFETs, but they required a gate drive around
12V to switch fully, and hence ran particularly warm in this configuration.
Technical Aid to the Disabled
Project 98-553
Revision AX3
10th October 1998
Supplier Part Description Cost Qty Price
Stock N/A PCB 4x2 Veroboard 3.25 0.50 1.62
Jaycar WH-5543 Heatshrink Tubing 1.95 0.08 0.16
Jaycar YX-2502 Fan 12V DC 0.2A 40mm USED 10.00 2.00 20.00
MicroZed IC 12C509-4 PIC12C509/04/P 8 pin RISC CPU 3.00 1.00 3.00
Jaycar PI-6452 Socket IC 8 pin DIP 1.05 1.00 1.05
Oatley 2SK2175 2SK2175 TO-220 Logic MOSFET 2.60 2.00 5.20
J V Spares 47382 Globe 12V 21W BC 1.50 2.00 3.00
Stock N/A Resistor 100 0.25W 0.04 1.00 0.04
Stock N/A Resistor 100 0.50W 0.15 1.00 0.15
Stock N/A Resistor 10K 0.25W 0.04 1.00 0.04
Stock N/A Capacitor 0.01uF Ceramic 0.40 2.00 0.80
Stock N/A Capacitor 10uF Tantalum 1.00 1.00 1.00
Jaycar RE-6230 Capacitor 1000uF Electrolytic 0.95 1.00 0.95
MicroZed IC 7805 LM7805 5V 1A Regulator 0.45 1.00 0.45
Jaycar ZR-1014 1N5404 3A 400V Diode 0.30 1.00 0.30
Jaycar ZR-1100 1N4148 Signal Diode 0.06 1.00 0.06
Jaycar ZR-1004 1N4004 1A 400V Diode 0.08 2.00 0.15
Jaycar HM-3180 10 Way PCB Terminal Strip 1.50 1.00 1.50
Jaycar PP-0524 2.5mm Bulkhead Socket 2.25 1.00 2.25
Jaycar PP-0512 2.5mm ID 5.5mm OD DC Plug 1.30 1.00 1.30
Jaycar PS-0123 3.5mm Insulated Socket 1.60 1.00 1.60
Jaycar MP-3018 13.8V DC 2A Non-reg Supply 42.95 1.00 42.95
Jaycar SF-2160 M205 2A Fuse 0.35 1.00 0.35
Jaycar SZ-2018 M205 Fuse Clips, PCB 1.40 0.25 0.35
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Total 87.58
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